346 A. W. GRABAr — PALEOZOIC CORAL RKEFS 



for the next 26.7 kilometers. Of forty-one slopes determined on the 

 Bahama islands, the lowest is degree, the highest 26 degrees 18 minutes. 

 Seventj^-five per cent of the slopes are below 10 degrees, while one-third 

 of the entire number of determined slopes fall below^ 5 degrees. 



On the Keeling islands, on the other hand, onh^ a few slopes below 

 10 degrees are recorded by Dietrich. Nearly half of the number lie 

 between 30 degrees and 43 degrees, one being as high as 63 degrees 21 

 minutes. In the section of one of these islands given by Darwin the 

 slope is at first a very gentle one, plunging at a distance from shore only, 

 at an angle of 45 degrees. On these islands Darwin also found that 

 below 20 fathoms depth the material of the bottom was coral sand, while 

 above 12 fathoms the bottom was free from sand. The sand brought up 

 from a depth of 200 to 300 fathoms was mainly of finely triturated frag- 

 ments, while shell fragments were rare. Along the outer margin of the 

 reefs breccias and conglomerates of coral fragments are found. These 

 are firmly cemented even in the growing reef, so that Darwin found diffi- 

 culty in chopping off fragments even with a chisel. 



Origin of the Coral Sand 



The production of the coral sand is partly due to the direct activities 

 of animals which feed on the calcareous organisms and partly to the 

 activities of the waves themselves. Fish feeding on living corals were 

 found by Darwin to have their intestines distended b}' small pieces of 

 coral and finely ground calcareous matter. This must pass daily from 

 them as the finest sediment. Worms, mollusks, Crustacea, and holo- 

 thurians also grind up rock. These organisms are numerous on ever}' 

 modern coral reef, where they find a rich feeding ground. The most 

 efficient among them are probably the Crustacea, and these are generally 

 credited with the production of most of the coral sand. 



In the Paleozoic reefs, however, the Crustacea probabl}' played a minor 

 part in the reduction of coral masses to sand and fine silt. While trilo- 

 bites may have been active in this respect to some extent, they can not 

 be regarded as possessing even in a fair measure the efficiency of the 

 modern decapods. Moreover, trilobite remains are not alwaj^s common 

 in these reefs. While, therefore, a part of the production of the coral 

 sand and mud is undoubtedl}^ due to these and to the other tj^pes of 

 animals mentioned, the greater part of the detrital material could not 

 have been produced by them. A very important factor, and the one 

 believed by Wiman to have caused the })roduction of most of the lime 

 sand of the Siluricbeds of Gotland, is the natural tendency of echinoderm 

 skeletons to become dissociated into their component plates on the death 



